Beta codes are tied to Bungie.net accounts, not whatever you have linked. The Beta will be given to whatever account you redeemed it on regardless of what accounts you have linked to it or to any other accounts. Changing what you have linked to an account will not take away your Beta access, no.

Beta Reservation Codes are associated with your B.net account, not the Playstation/Xbox name you have linked to your account. Just get the actual Beta Code from the 2nd account when it's time and download it to your console.

Just make sure you can check the email for the B.net account associated with your code. Bungie will email you the code that you can enter on your console, and you don't have to use it on the account you have linked.